That's because Bill Kristol is no conservative, he is a neo-conservative/New World Order Globalist. Having open borders is one of his top issues. It's the way elites acquire more and more wealth.
I've been saying for years that neo-cons are liberals who are not pansies on foreign affairs. I've gotten responses ranging from "there is no such thing as neo-conservatism" to "you're a Jew-hater".